This invention relates generally to double conversion television tuning systems and in particular to a double conversion tuning system for use with digital television (TV) signals.
The recently proposed Zenith/AT&T Digital Spectrum Compatible High Definition Television System (DSC/HDTV) transmits wide band television information that is digitally encoded. Processing of the digital information imposes severe restrictions on the amount of phase noise that may be tolerated in the tuning system. It is critical that the tuning system impose a minimal amount of phase noise on the digital information which, lacking the redundancy of conventional television signals, demands accurate detection and decoding. A double conversion type tuning system appears to be the most desirable for digital HDTV signals, although conventional NTSC double conversion tuning systems do not exhibit sufficiently low phase noise characteristics to be entirely satisfactory. This is due to both the low "Q" of the varactor diode-tuned tank circuit in the first local oscillator of the double conversion tuning stage and the narrow band phase locked loop (PLL) of the synthesizer. The arrangement of copending application Ser. No. 815,721 above remedies the first part of the problem, i.e. low Q of the tuned circuits. The latter part of the problem is the subject of the present invention. Thus the present invention describes a double conversion tuning system that meets both the criteria of low phase noise and tuning accuracy.